Modems like Digital Subscriber Line (DSL) modems are normally connected to a device for providing access to a data communication network. Such a device is typically a Digital Subscriber Line Access Multiplexer (DSLAM) and the communication network may be the Internet. One particular type of standard that may be of interest is Very-high-speed Digital Subscriber Line 2 (VDSL2).
When being connected in this way the modems are connected to the device via separate communication lines, typically conductor pairs made of copper. These lines are furthermore often bundled together as a set of connection lines that is provided in a common cable leading to the device. This means that the communication lines are often placed very close to each other. There is in this regard a problem in that a communication line may be subject to crosstalk from one or more neighboring communication lines. This limits the communication capability in that the rate at which data is transmitted is limited.
There has in recent years evolved techniques for reducing the influence of crosstalk, for instance in relation to VDSL2. VDSL2 has been standardized by the Telecommunication Standardization Sector of the International Telecommunications Union (ITU-T) in recommendation G.993.2.
ITU-T has issued a further recommendation G.993.5, specifying vectoring for VDSL2. Vectoring is a technique for Far-end crosstalk (FEXT) cancellation where the transmission and/or reception on communication lines in a cable where VDSL2 is used are jointly processed. In the downstream direction pre-coding is used, which pre-distorts the transmitted signals in such a way so that the crosstalk into other lines is cancelled as the signal propagates along the cable.
In the upstream direction the received signals are post-processed to cancel the FEXT. The VDSL2 FEXT crosstalk is the static noise that most severely limits the performance of VDSL2 systems. The vectoring recommendation provides a way to estimate the FEXT channel in both downstream and upstream and utilize the estimated crosstalk channel to cancel the crosstalk.
This technique provides a significant improvement for DSL modems.
However, as these new types of modems supporting vectoring are being introduced. There still exist older types of modems, often denoted legacy modems, which operate according to previous versions of the VDSL2 standard and do therefore not support vectoring. These older and newer types of modems will most probably have to co-exist for a long period of time.
Furthermore, in order for the vectoring to be able to function properly it is important that the legacy modems either are provided with vectoring support functionality or are provided with functionality that ignores the vectoring so as not to interfere with it.
This is not so simple to do because the modems may be the property of end users, i.e., they may be Customer-premises equipment (CPE). These end users may very well not want any changes to be made to their modems. This means that an update or retrofit of legacy modems may very well not be possible to make.
This thus means that there is a need for a solution to the problem of stopping older types of modems interfering with the vectoring supported by new types of modems.